1. Field of the Invention
This invention relates to packaging of semiconductors in general, and in particular, to a very thin, small outline, thermally enhanced semiconductor package having a coined leadframe that provides a die pad with an exposed surface and features that lock the leadframe more securely to the plastic body of the package.
2. Description of the Related Art
Integrated circuits (“ICs”) are formed on a single die, or “chip,” cut from a semiconductor wafer containing a large number of identical dies. The dies are relatively small and fragile, are susceptible to harmful environmental elements, particularly moisture, and generate a relatively large amount of heat in a relatively small volume during operation. Accordingly, ICs must be packaged in affordable, yet robust packages that protect them from the environment, enable them to be reliably mounted to and interconnected with, for example, a printed circuit board (“PCB”) populated with associated electronic components, and to effectively dissipate the heat they generate during operation.
Leadframe types of semiconductor packages are well known and widely used in the electronics industry to house, mount, and interconnect a variety of ICs. A conventional leadframe is typically die-stamped from a sheet of flat-stock metal, and includes a plurality of metal leads temporarily held together in a planar arrangement about a central region during package manufacture by a rectangular frame comprising a plurality of expendable “dam-bars.” A mounting pad for a semiconductor die is supported in the central region by “tie-bars” that attach to the frame. The leads extend from a first end integral with the frame to an opposite second end adjacent to, but spaced apart from, the die pad.
During package manufacture, an IC die is attached to the die pad. Wire-bonding pads on the die are then connected to selected ones of the inner ends of the leads by fine, conductive bonding wires to convey power, ground, and signals between the die and the leads.
A protective body of an epoxy resin is molded over the assembly to enclose and seal the die, the inner ends of the leads, and the wire bonds against harmful environmental elements. The rectangular frame and the outer ends of the leads are left exposed outside of the body, and after molding, the frame is cut away from the leads and discarded, and the outer ends of the leads are appropriately formed for interconnection of the package with other, associated componentry.
In a variant of the above configuration, viz, a “land grid array” (“LGA”), or a “leadless chip carrier” (“LCC”) package, the outer portions of the leads are removed entirely from the package, and a terminal, or “land,” is provided on the lower surface of the leads and exposed through the lower surface of the body for mounting and interconnection of the package to a PCB. In yet another variation, the die pad is “down-set” relative to the plane of the leads such that its lower surface is exposed through the lower surface of the body for enhanced dissipation of heat from the die.
While the foregoing prior art package configurations provide a reasonable compromise between packaging cost and performance, they also include some recognized problem areas where there is a long-felt need for improvement. One of these relates to the problem of making reliable wire bonds to leads that have been made extremely narrow to accommodate an extremely fine lead pitch. In particular, as package sizes decrease, lead densities remain the same or even increase. In response, leads are made much narrower so that they can be placed closer together. At some limiting width and pitch of the leads, the leads become so narrow and close together that it is difficult to make wire bonds to them reliably. It is therefore desirable to provide a leadframe design that can accommodate very narrow, closely pitched leads, yet one in which reliable wire bonds can be made to the leads.
Another problem relates to delamination of the leadframe components from the plastic package body, and the attendant problem of penetration of the package by moisture. In particular, the various parts of a semiconductor package experience greatly different amounts of thermal expansion and contraction with temperature changes due to the relatively large differences in the coefficients of thermal expansion of their respective materials, e.g., metal, epoxy resin, and silicon.
As a result, the leadframe components can become delaminated from the package body with temperature cycling of the package during manufacture or operation. Where delamination occurs at a boundary of the package body, a microscopic crack is created for the penetration of the package by moisture. This penetration can wreak a two-fold assault on the package: First, the moisture can corrode any metallizations present in its path, resulting in subsequent current leakage through the corrosive path; second, the moisture can expand and contract with temperature cycling of the package, resulting in further propagation of the cracks into the package, and hence, further penetration of the package by moisture. It is therefore desirable to provide a leadframe design that more securely locks the leadframe components to the plastic body of the package, thereby effectively reducing both the amount of delamination of the leadframe from the body and the resulting penetration of the body by moisture.